The Isle of Wight Randonnee is a 100km circular route around the IoW with a shorter option also provided. The event is free to enter with donations suggested and a number of checkpoints which also act as start points so the 3,000 entrants can be staggered. It is not competitive and runs 9am to 6pm. In theory you could treat it as a day trip from London if you are fast enough though I have not worked out the train and ferry timings for that.
The graphic below shows the extra miles getting to a start at Wooten.
The organisation running the event is now the Mountbatten Isle of Wight charity and information - including registering interest - can be found at https://www.mountbatten.org.uk/randonnee2026
You can stay at the cycle-friendly Premier Inn at Newport which means a 25 minute cycle to Wootton or East Cowes for one of the starts; subject to confirmation there are 5 starting points, East Cowes, Wootton, Bembridge, Ventnor and Freshwater/Yarmouth.
A PDF of the previous route can be found at:
https://www.cycleisland.co.uk/images/map_1.pdf though check the latest information from Mountbatten IoW and, critically, what direction is being taken.
There are options for getting to the Isle of Wight:
Take the train to Southampton then the Red Funnel ferry to East Cowes; these run every hour and the crossing takes an hour.
Take the train to Portsmouth Harbour then the ferry to Fishbourne; these run every 40 minutes and the crossing takes 45 minutes. Portsmouth Harbour to Ryde is also possible.